Method for purifying a melamine melt

ABSTRACT

The invention relates to a method for producing pure melamine in a high-pressure method. According to said method, in the first stage, the melamine melt is brought into contact with hot NH 3  and NH 3  from the second stage and in the second step is brought into contact with cold NH 3  in such a way that it is cooled to a temperature, which is 1–30° C. above the pressure-dependent melting point of the melamine, before being optionally rested in a third stage and subsequently treated in various ways.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the preparation of pure melamine by pyrolysisof urea in a high-pressure process and purification of the resultingmelamine melt by divided stripping.

2. Description of the Prior Art

In high-pressure processes for preparing melamine, urea is reacted in anendothermic liquid-phase reaction to give melamine. In this reaction, 3mol of CO₂ and 6 mol of NH₃ are formed per mole of melamine, and theliquid melamine contains, depending on pressure and temperatureconditions in the reactor, additional varying amounts of dissolved NH₃and CO₂ and also high molecular weight and low molecular weightby-products and unreacted urea.

It is known from U.S. Pat. No. 3,116,294 that the CO₂ can be removedfrom the crude melamine by countercurrent stripping of a crude melaminemelt with gaseous NH₃. In WO 00/21940, it is stated that the strippingis advantageously carried out in a column which is filled with liquidmelamine and not in a gas-filled column. Furthermore, it is possible tocool the melt in the bubble column to a temperature which is 5–20° C.above the melting point of melamine.

DETAILED DESCRIPTION OF THE INVENTION

Unexpectedly, a process which makes it possible to obtain a higherpurity of the melamine by divided NH₃ introduction into the stripper hasnow been found.

The invention accordingly provides a process for preparing pure melaminein a high-pressure process by pyrolysis of urea, which is characterizedin that the melamine melt formed is fed into a stripping unit in whichthe melamine melt is, in countercurrent,

-   -   in a first stage brought into contact with hot fresh gaseous NH₃        and additionally with NH₃ from the second stage, with the        temperature of the melt remaining the same or becoming higher,    -   in a second stage brought into contact with cold gaseous NH₃ in        such a way that it is cooled to a temperature which is 1–30°        above the pressure-dependent melting point of melamine and, if        desired,    -   in a third stage left to rest for 10 minutes-10 hours,    -   whereupon the melamine melt is worked up in any desired way.

To carry out the process of the invention, urea is reacted at atemperature of 325–450° C., preferably 350–425° C., and a pressure of50–450 bar, preferably from 50to 250 bar, to form liquid melamine andoffgas. To avoid formation of by-products or, depending on theconstruction of the melamine reactor, to improve mixing in the reactor,excess NH₃ gas up to 10 mol of NH₃, preferably up to 2 mol of NH₃, permole of urea is introduced into the reactor.

After the process of the invention, it is not necessary to feed thereaction mixture formed in the reactor into a separator and thereseparate the liquid phase, viz. the melamine melt, completely from thegaseous phase, viz. the offgases.

It is sufficient to take off the offgases at the top of the reactor andto pass the liquid phase which still contains amounts of dissolvedoffgases directly to the stripping unit.

The offgases taken off at the top of the reactor, which comprise gaseousNH₃, CO₂ and small amounts of gaseous melamine, are, if desired togetherwith the offgases from further high-pressure parts of the melamineplant, which likewise comprise gaseous NH₃, CO₂ and small amounts ofgaseous melamine, fed to a urea scrubber. In the urea scrubber, there isa urea melt which scrubs out the gaseous melamine present in the hotoffgas and at the same time becomes hotter, while the offgas is freed ofmelamine and cooled. The preheated, melamine-containing urea melt isthen fed into the melamine reactor and converted into melamine.

The melamine melt leaving the reactor additionally contains varyingamounts of dissolved NH₃ and CO₂ and also relatively high molecularweight and low molecular weight by-products and unreacted urea. The CO₂and the by-products and the unreacted urea should be removed ascompletely as possible from the melamine melt. According to theinvention, this is achieved by divided stripping in a stripping unit inwhich the melamine melt is, in the first stage, stripped incountercurrent with hot fresh NH₃ and additionally with hot NH₃ from thesecond step, with the temperature of the melt remaining the same orbeing increased, and the melamine melt which has been prepurified inthis way is, in the second stage, stripped with cold gaseous NH₃ in sucha way that the melt is cooled to a temperature which is only just abovethe pressure-dependent melting point of melamine. Depending on theconstruction of the plant, this temperature is 1–30° C., preferably1–20°, particularly preferably 1–10° C., above the pressure-dependentmelting point of melamine.

The melamine melt is under an NH₃ pressure of 50–450 bar, preferably thereactor pressure. However, it is also possible to carry out strippingsuccessfully at a pressure significantly below the reactor pressure. Thetemperature of the melamine melt entering the stripping unit is, in apreferred embodiment, about the same as the reactor temperature.However, it is also possible to increase or reduce the temperature. Tocarry out the stripping in the first stage, fresh ammonia having atemperature which is about the same as the temperature in the meltpresent in the first stage is introduced. However, it is also possibleto feed in NH₃ having a somewhat higher temperature, so that the melttemperature in the first stage is increased overall.

At the same time as the fresh hot NH₃ is fed in, the now heated NH₃obtained after passage through the second stage is also introduced intothe first stage, so that the total amount of NH₃ used in the strippingunit at the high temperature necessary for optimal stripping isavailable for the stripping process in the first stage.

The inlet for the fresh hot NH₃ is present in any zone of the strippingunit in which the melamine melt which has been cooled in the secondstage has reached approximately the entry temperature of the melaminemelt into the first stage.

The amounts used, temperatures and particular structure of the strippingunit determines the position of the zone in which virtually no morecooling of the melamine melt takes place and in which the first stagecommences and the fresh hot NH₃ is introduced. The fresh hot NH₃ gas canbe introduced at one or more points.

The introduction of the cold NH₃ into the second stage is effected atthe bottom of the stripping unit in an amount and at a temperature whichcools the melamine melt to a temperature which is just above itsrespective melting point. However, it has to be ensured that thetemperature does not drop below the melting point in the event of anyfluctuation in operation, so that melamine crystallizes.

The temperature of the cold NH₃ is in the range 150–300° C., preferably150–200° C., and the amount of cold NH₃ necessary is dependent on theentry temperature of the melamine melt, on the throughput, on the sizeand construction of the stripping unit, and can accordingly vary withina wide range.

During passage through the second stage, the cold NH₃ takes up thequantity of heat removed during cooling of the melamine melt and is inthe process heated itself. In this second stage, relatively highmolecular weight by-products, in particular, are removed.

In a further embodiment of the invention, there is room beneath thesecond stage for a third stage in which the melamine melt which has beenbrought to a temperature just above the melting point of melamine isleft to rest under the prevailing NH₃ pressure. An additional agingeffect can be achieved in this way.

The pure melamine melt obtained at the outlet of the second or thirdstage can be worked up further in any desired way and solidified, forexample by depressurization of the melt, by solidification in afluidized bed, by quenching with water, with liquid or gaseous ammonia,or by sublimation and subsequent desublimation from the gas phase.

The process of the invention is suitable for all types of gas-liquidcolumns. A wide variety of packing, for example perforated plates, sievetrays, valve trays or ordered packings such as Sulzer packing, can beused.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a stripping unit in which A is the firststage of the stripping unit, B is the second stage of the strippingunit, 1 is the melamine melt entering the first stage, 2 is the melaminemelt leaving the second stage, 3 is hot gaseous NH₃, 4 is cold NH₃ and 5is outflowing NH₃.

Example 1 below shows that the divided addition according to theinvention of NH₃ at different temperatures gives, at the same amount ofNH₃ for stripping, melamine of higher purity than is obtained in thecase of undivided NH₃ addition at constant temperature at the bottom ofthe reactor as in comparative example 1. Comparative example 2 showsthat if the column has a temperature profile between inlet and outletfor the melamine melt, it is not possible to introduce the amount of“cold” NH₃ necessary for producing pure melamine, since the temperatureof the melamine at the outlet of the stripping unit can then no longerbe kept at the required temperature. Melamine having a lower qualitystill is obtained.

EXAMPLE 1 Divided NH₃ Addition

4 kg of melamine/hour having a temperature of 370° C. were introduced atthe top into a column which had a length of 1 meter and a diameter of 8cm, was packed with Sulzer packing and was under an NH₃ pressure of 180bar. The melamine had a content of oxygen-containing components (CO₂,ammeline, ammelide, ureido-melamine and isocyanic acid) of 2.9% byweight. 0.43 kg of NH₃ at a temperature of 160° C. was introduced at thebottom of the column, and 2.9 kg/h of NH₃ at a temperature of 370° C.were introduced at the beginning of the temperature range of themelamine melt of 370° C.

At the bottom of the column, where the temperature is 340° C., melaminewas obtained at a purity of 99.6% by weight and a content ofoxygen-containing components of 0.21% by weight.

COMPARATIVE EXAMPLE 1 Undivided NH₃ Addition

As in example 1, 4 kg of melamine having a content of oxygen-containingcomponents of 2.9% by weight and a temperature of 370° C. wereintroduced at the top of the column. However, the total amount of NH₃ of3.33 kg/h was introduced at the bottom of the reactor and at atemperature of 370° C.

The purity of the melamine melt obtained at the bottom of the reactorwas 99.0% by weight, and its content of oxygen-containing components was0.22% by weight.

COMPARATIVE EXAMPLE 2 Undivided NH₃ Addition

As in example 1, 4 kg of melamine/having a content of oxygen-containingcomponents of 2.9% by weight and a temperature of 370° C. wereintroduced at the top of the column. NH₃ having a temperature of 330° C.was introduced at the bottom of the stripping column until thetemperature at the bottom of the column was 340° C.

The content of oxygen-containing components in the melamine meltobtained at the bottom of the reactor was 0.45% by weight, and thepurity of the melamine was 99.2% by weight.

1. A process for preparing pure melamine in a high-pressure process bypyrolysis of urea, wherein the melamine melt formed is fed into astripping vessel having at least a first and a second separate gas inletin which the melamine melt is, in countercurrent, in a first stagebrought into contact with hot fresh gaseous NH₃ from the second stage,with the temperature of the melt remaining the same or becoming higher,whereby the gaseous NH₃ is injected at the first gas inlet into thefirst stage, in a second stage brought into contact with cold gaseousNH₃ in such a way that it is cooled to a temperature which is 1–30° C.above the pressure-dependent melting point of melamine, whereby thegaseous NH₃ is injected at the second gas inlet into the second stage,whereupon the melamine melt is worked up in any desired way.
 2. Theprocess as claimed in claim 1, wherein the melamine melt after thesecond stage is left to rest for 10 minutes-10 hours in a third stage.3. The process as claimed in claim 1, wherein the melamine melt is inthe second stage brought into contact with cold gaseous NH₃ in such away that it is brought to a temperature which is 1–20° C. above thepressure-dependent melting point of melamine.
 4. The process as claimedin claim 2, wherein the melamine melt is in the second stage broughtinto contact with cold gaseous NH₃ in such a way that it is brought to atemperature which is 1–20° C. above the pressure-dependent melting pointof melamine.
 5. The process as claimed in claim 1, wherein the melaminemelt is in the second stage brought into contact with cold gaseous NH₃in such a way that it is brought to a temperature which is 1–10° C.above the pressure-dependent melting point of melamine.
 6. The processas claimed in claim 1, wherein the melamine melt comes directly from thereactor.
 7. The process as claimed in claim 1, wherein the temperatureof the melamine melt in the first stage is the same as the reactortemperature.
 8. The process as claimed in claim 1, wherein theintroduction of hot fresh gaseous NH₃ into the first stage is effectedat a plurality of points.
 9. The process as claimed in claim 1, whereinthe temperature of the cold gaseous NH₃ introduced in the second stageis 150–300° C., preferably 150–200° C.